The present invention relates to a hydraulic shock absorber which is suitably mounted on a suspension apparatus for a vehicle, such as an automobile.
Generally, a cylinder-type hydraulic shock absorber mounted on a suspension apparatus for a vehicle, such as an automobile, comprises: a cylinder in which a hydraulic fluid is sealably contained; a piston portion which is slidably fitted into the cylinder and connected to a piston rod; and a damping force generating mechanism provided in the piston portion. The damping force generating mechanism includes a hydraulic fluid passage, an orifice, a disk valve, etc. In the damping force generating mechanism, a flow of hydraulic fluid in the hydraulic fluid passage, which is generated due to a sliding motion of the piston in the cylinder according to a stroke of the piston rod, is controlled by means of the orifice and the disk valve, to thereby generate a damping force. In a low-speed region of the piston speed, a damping force is generated by means of the orifice. In a high-speed region of the piston speed, the disk valve opens by being deflected, to thereby prevent an excessive increase in damping force.
In the above-mentioned related art of a hydraulic shock absorber, the damping force in the low-speed region of the piston speed is dependent on a flow path area of the orifice, and the damping force in the high-speed region of the piston speed is dependent on a predetermined valve-opening pressure of the disk valve. Therefore, it is difficult to set damping force characteristics with a high degree of freedom.
The above-mentioned related art is disclosed in, for example, Japanese Patent Application Public Disclosure No. HEI 3-113139. The hydraulic shock absorber disclosed in this document comprises a back-pressure chamber and a relief valve which are formed on a rear side of the disk valve. When a pressure in the back-pressure chamber reaches a predetermined level, the pressure is relieved through the relief valve. With this arrangement, part of the hydraulic fluid is introduced through an inlet fluid passage into the back-pressure chamber, and is released through a downstream-side orifice into a downstream-side chamber. Thus, by applying the back pressure in the back-pressure chamber to the disk valve in a valve-closing direction and controlling this back pressure in the back-pressure chamber, a degree of freedom for setting damping force characteristics is increased.
In the above-mentioned related art of a hydraulic shock absorber comprising a back-pressure chamber, to effectively apply the back pressure to the disk valve, it is considered to increase an area of the back-pressure chamber (this structure is not known). With this structure, an outer diameter of the back-pressure chamber becomes larger than a diameter of a valve seat portion for the disk valve. In this case, during a reverse stroke (an extension stroke for a compression-side disk valve, and a compression stroke for an extension-side disk valve), the disk valve receives, at an outer circumferential portion thereof corresponding to a difference between the outer diameter of the back-pressure chamber and the diameter of the seat portion, an increased pressure in an upstream-side chamber in a valve-opening direction.
Further, when the flow path area of the downstream-side orifice is set to be smaller than the flow path area of the inlet fluid passage, the pressure in the back-pressure chamber for the compression-side disk valve is not generated during the extension stroke of the piston rod, and the pressure in the back-pressure chamber for the extension-side disk valve is not generated during the compression stroke. In this case, under a force acting in a valve-opening direction based on the above-mentioned difference in diameter, the disk valve opens during the reverse stroke, so that damping force becomes unstable.